1. Field of the Invention
The present invention concerns a method for controlling a magnetic resonance system having a radio-frequency antenna structure and a number of individually controllable transmission channels, in which radio-frequency signals are respectively emitted in parallel via the transmission channels to generate a desired radio-frequency field distribution in at least one specific volume region within an examination volume of the magnetic resonance system. The invention also concerns a magnetic resonance system for implementation of such a method.
2. Description of the Prior Art
Magnetic resonance tomography is a wide-spread technology to acquire images of the inside of the body of a living examination subject. In order to acquire an image with this method the body or body part of the patient or test subject to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field which is generated by a basic field magnet of the magnetic resonance system. Rapidly switched gradient fields that are generated by gradient coils are superimposed on this basic magnetic field for spatial coding during the acquisition of the magnetic resonance images. Moreover, radio-frequency pulses of a defined field strength (known as the “B1 field”) are radiated with radio-frequency antennas into the examination subject. The nuclear spins of the atoms in the examination subject are excited by these radio-frequency pulses such that they are deflected from their equilibrium state (parallel to the basic magnetic field) an amount known as an “excitation flip angle”. The nuclear spins then precess around the direction of the basic magnetic field. The magnetic resonance signals thereby generated are acquired by radio-frequency acquisition antennas. The magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals.
Different images can be generated by the emission of different pulse sequences that respectively include a series of radio-frequency pulses and gradient pulses precisely correlated to one another. A method to monitor and optimize the pulse sequences is explained in U.S. Pat. No. 5,519,320.
The tomography scanner typically has an antenna structure permanently installed in the housing thereof for the emission of the required radio-frequency pulses in the patient positioning region. This radio-frequency antenna is also known as a body coil. It has (for example in the typically employed birdcage structure) a number of conductor rods arranged around the patient space and running parallel to the main field direction, which are connected with one another by ferrules (annular conductors) at the ends of the coil. As an alternative to this, however, there are also other antenna structures permanently installed in the housing such as, for example, saddle coils. Conventional magnetic resonance systems have essentially only one transmission channel for emission of the B1 field, meaning that there is only one transmission line that leads from the radio-frequency antenna to the antenna structure. Insofar as the antenna (such as, for example, a birdcage antenna) is fashioned such that a circularly-polarized field can be emitted, the radio-frequency signal from the radio-frequency antenna is split via a hybrid module into two signals that are offset from one another by 90° in their phase. The two signals are then fed into the antenna structure at precisely defined connection points via two transmission lines. The distribution of the B1 field is unalterably fixed by the splitting to the two signal paths with the phases of 0° and 90° and cannot be adapted to the current conditions of the present measurement. Moreover, local coils can also be used that are arranged directly on the body of the patient, but these coils are normally used only as acquisition coils.
The generation of the radio-frequency pulses or radio-frequency pulse sequences for generation of the desired B1 field initially ensues via generation of a digital signal on the basis of which an RF signal is then generated. This is schematically shown in FIG. 1. The generation of the digital signal DS ensues here in a digital pulse generator 3′ in a channel control unit 2′. This digital signal DS already exhibits the desired pulse shape of the radio-frequency pulse or the radio-frequency pulse sequence to be generated. The digital signal DS is mixed with the desired RF carrier frequency RFT in a modulator 7′. The carrier frequency RFT depends on the magnetic field as well as on the desired data acquisition, i.e. on which nuclear spins should be excited, such as whether H1, F19, P31, NA23, C13 or other protons will be excited. Given a 3 Tesla basic magnetic field and an H1 excitation (desired in most cases), the carrier frequency is, for example, 123 MHz. The radio-frequency signal RF generated in the modulator essentially exhibits all characteristics of the radio-frequency signal to be emitted, meaning that it has the exact pulse shape and the required carrier frequency. Only the power of the yet unamplified signal is relatively low. This signal RF is therefore also frequently designated as a radio-frequency small (low-level) signal.
This radio-frequency small signal RF is then sent to the radio-frequency antenna structure 10 via a signal path, wherein it is normally amplified in an RFPA (radio-frequency power amplifier) 8′. After this amplification, in order to monitor the radiated radio-frequency power in order to ensure the compliance with the SAR limit values (SAR=specific absorption rate) a signal portion is typically extracted (tapped) in a directional coupler 9R′ and monitored in a radio-frequency monitoring device 9′, often also designated as an RFSWD (radio frequency safety watch dog).
Imperfections in the signal path (in particular in the radio-frequency power amplifier 8′ but also in the feed lines and further employed conductors in the signal path) lead to small but unavoidable phase and amplitude distortions of the radio-frequency signal RF′ that is ultimately fed into the antenna structure 10. This means that the radio-frequency signal RF′ actually fed into the antenna structure 10 no longer exactly exhibits the original characteristics predetermined by the digital signal DS. The alterations can be described by the characteristic curves KL (the term “curve” being used in the mathematical sense as also encompassing a linear characteristic) of the amplitude and phase response over the signal path (or the radiated, distorting portion of the signal path, the RFPA 8′). An example for a characteristic amplitude line is shown in FIG. 2A. The magnitude of the amplitude Aout of the output signal that exits the signal path and is fed into the antenna structure 10 is plotted over the magnitude of the amplitude Ain of the input signal arriving from the modulator 7′. An ideal characteristic curve KAi would lead to no distortions. This would be a diagonal line in the diagram. A distorted real characteristic curve KAr in practice more likely exists, as is shown in FIG. 2A as an example. This means that the output amplitude Aout can sometimes be too high and sometimes too low, depending on the amplitude of the input amplitude Ain. The same applies for the phase response which is shown in FIG. 2B. Here the phase φout of the output signal after the signal path is plotted over the phase φin of the input signal before the signal path. Here the ideal characteristic curve Kφi would again be a diagonal, but the real characteristic curve Kφr actually exhibits a different curve.
In order to solve this problem, in U.S. Pat. No. 5,053,709 it is suggested to regulate the amplification factor in a control loop such that the characteristic amplification line is linear. Alternatively, what is known as a characteristic curve correction has been used. This is shown in FIG. 3. FIG. 3 shows essentially the same design as in FIG. 1. In FIG. 3, however, it is schematically shown that the characteristic curve KL is measured and the digital signal DS is pre-distorted in a characteristic curve correction unit 4′ on the basis of this characteristic curve, meaning that a distorted digital signal DS′ is generated therefrom. The later distortion of the radio-frequency small signal thus is compensated in the phase response, so ultimately a radio-frequency signal RF′ that precisely corresponds to the requirements of the digital signal DS arrives at the antenna. For this purpose, it is sufficient for the characteristic curve to be measured once at the manufacturer upon the production of the apparatus. This is then stored in a file and can be applied by the characteristic curve correction unit in each pulse generation. Thus no change of the stored characteristic curve is therefore necessary during the lifespan of the apparatus except for the cases in which components within the signal path are exchanged.
A further problem of the signal adulteration that particularly occurs in newer magnetic resonance systems with basic magnetic field strengths greater than three Tesla is that considerable eddy currents can be induced in the patient upon radiation of the radio-frequency pulses. The actual homogeneously radiated B1 field in the examination volume is more or less strongly distorted as a consequence. The influence of the patient body on the B1 field is dependent on, among other things, the size of the patient and the proportions of the individual tissue types. For example, for a very corpulent patient a circularly-polarized magnetic field is strongly distorted into an elliptical field. By contrast, for thinner patients this distortion is not so strong. In individual cases this can lead to the situation that a reliable magnetic resonance data acquisition is problematic in specific body regions of the patient, and unusable results can occur.
One approach to address this problem is the use of antenna configurations known as transmission arrays (TX arrays) for adjustment of arbitrarily shaped B1 fields. Signals for RF pulses are supplied in parallel (i.e. simultaneously or with slight temporal offset) to the radio-frequency antenna structure via different transmission channels. An example of this is explained in DE 101 24 465 A1, which describes an antenna with a number of separately controllable antenna elements. This means that each transmission channel has a separate antenna element. The radio-frequency pulses emitted with different amplitudes and phases, which radio-frequency pulses are sent out by the individual antenna elements, then superimpose in the examination volume and form the desired individually adjustable B1 field distribution. Various feed lines connected to a common antenna structure alternatively can be supplied by individually controllable transmission channels, so the superimposition of the RF pulses already ensues within the antenna structure.
One possibility to modify the B1 field in this manner is known as “static B1 shimming”, analogous to “static magnetic field shimming” of the basic magnetic field. Corrections are impressed on the B1 field by amplitude and phase control elements that are installed in the individual radio-frequency channels of the system. These must then respectively be adjusted such that the desired B1 distribution is achieved. Such an approach is explained in the aforementioned DE 101 24 465 A1. The insertion of suitable high-precision phase control elements and amplitude control elements into every transmission channel, however, is expensive. A number of feed lines are additionally necessary in order to control the respective control elements. Another possibility disclosed in DE 101 24 465 A1 for adjustment of the corrections is to individually calculate every radio-frequency pulse of a sequence in advance. This means that each of the radio-frequency pulses emitted in parallel has a different appearance with regard to amplitude and phase. The sequence programmer who programs the necessary pulse sequences for the large number of measurement protocols in advance therefore must not only program a pulse sequence for the application in such systems, but also must expend significant effort for each of the individual transmission channels (for example eight, sixteen or more transmission channels) so as to generate individual radio-frequency pulses for every possible pulse sequence in order to achieve the desired result. This represents a considerable degree of complexity for the programmer. In addition, other corrections are required depending on the load of the MR apparatus, meaning that a number of different radio-frequency pulse sequences for a number of examination situations would then have to be specially provided for the respective apparatus.